Naturally occurring and synthetic zeolites have been demonstrated to exhibit catalytic properties for various types of hydrocarbon conversions. Certain zeolites are ordered porous crystalline aluminosilicates having definite crystalline structure as determined by X-ray diffraction studies. Such zeolites have pores of uniform size which are uniquely determined by structure of the crystal. The zeolites are referred to as "molecular sieves" because the uniform pore size of a zeolite material may allow it to selectively adsorb molecules of certain dimensions and shapes.
By way of background, one authority has described the zeolites structurally, as "framework" aluminosilicates which are based on an infinitely extending three-dimensional network of AlO.sub.4 and SiO.sub.4 tetrahedra linked to each other by sharing all of the oxygen atoms. Furthermore, the same authority indicates that zeolites may be represented by the empirical formula EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.YH.sub.2 O
In the empirical formula, x is equal to or greater than 2, since AlO.sub.4 tetrahedra are joined only to SiO.sub.4 tetrahedra, and n is the valence of the cation designated. M. D. Breck, ZEOLITE MOLECULAR SIEVES, John Wiley & Sons, New York p. 5 (1974). In the empirical formula, the ratio of the total of silicon and aluminum atoms to oxygen atoms is 1:2. M was described therein to be sodium, potassium, magnesium, calcium, strontium and/or barium, which complete the electrovalence makeup of the empirical formula.
The prior art describes a variety of synthetic zeolites. These zeolites have come to be designated by letter or other convenient symbols, as illustrated by the zeolite. The silicon/aluminum atomic ratio of a given zeolite is often variable. Moreover, in some zeolites, the upper limit of the silicon/aluminum atomic ratio is unbounded. ZSM-5 is one such example wherein the silicon/aluminum atomic ratio is up to infinity. U.S. Pat. No. 3,941,871, reissued as RE. 29,948, discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added aluminum and exhibiting the X-ray diffraction pattern characteristic of ZSM-5.
Crystalline silicate ZSM-5 and its conventional preparation are taught by U.S. Pat. No. 3,702,886, the entire disclosure of which is incorporated herein by reference. It has a distinctive X-ray diffraction pattern which identifies it from other known crystalline silicates which is also described in U.S. Pat. No. 3,702,886, the entire contents pertaining to said X-ray. A crystalline silicate composition having the structure of ZSM-5 is taught in U.S. Pat. RE. No. 29,948, the entire disclosure of which is incorporated herein by reference. ZSM-5 is a zeolite which has a Constraint Index of 1 to 12. The method of determining Constraint Index and other zeolites having a Constraint Index of 1 to 12 is described by V. J. Frilette et al. Journal of Catalysis, Vol. 67, pp. 218-222 (1981).
The foregoing zeolites are included in a class of zeolites sometimes referred to as medium pore-size zeolites. The pore sizes of these zeolites range from about 5 to about 8 Angstroms.
Another class of zeolites sometimes referred to as large pore zeolites are zeolites whose pore sizes are greater than those of the medium pore zeolites, for example, greater than about 7 Angstroms. Typical examples of these are zeolites X, Y, L, A and Beta.
U.S. Pat. No. 4,590,323, related to aromatizing alkanes, describes titanium oxide modifier for zeolites. Inclusion of elements in addition to silicon, aluminum and oxygen can alter the properties of zeolites in specific catalytic conversions, for which they are used. The exact silica:alumina ratio of a zeolite can alter its properties in catalysis. In some zeolites, the silica:alumina ratio can be controlled during synthesis. Zeolite silica:alumina ratio can be varied by treatment of the as-synthesized zeolite. In N. W. Anderson et al. "Zeolites Treated with Silicon Tetrachloride Vapor", J. Chem Soc., Faraday Trans. 1, Part 1, Vol. 82, p. 1449-1469 (1986), dealumination of zeolites is described. Also see G. W. Skeels et al. ZEOLITE CHEMISTRY-"Substitution of Silicon for Aluminum in Zeolites via Reaction with Aqueous Fluorosilicate", 16th Inter. Zeolite Conf., Reno, p. 87-96 (1983) and H. K. Berger et al., ed. B. Imelik et al., "A New Method for the Dealumination of Faujasite-Type Zeolites", p. 203 (1980). In Berger et al., "Preparation of High-silica Faujasites by Treatment with Silicon Tetrachloride", J. Chem. Soc., Faraday Trans. 1, 81, p. 2889-2901 (1985).
Various patents describe inclusion of elements other than silicon and aluminum in the preparation of zeolites. Cf. U.S. Pat. No. 3,530,064, U.S. Pat. Nos. 4,208,305 and 4,238,318 describe the preparation of silicates in the presence of iron.